1. Field of the Invention
The invention concerns a or, focus adjustment apparatus that tracks the direction of movement of an object from a plurality of focus detection areas disposed within the photographic screen of a camera. More particularly, the invention relates to a focus adjustment apparatus that performs the tracking of focus detection areas in an efficient order.
2. Description of Related Art
In recent years conventional cameras automatically adjust the focus by following the object image moving on the photographic screen. Japanese Laid-Open Patent No. 1-288816, the subject matter of which is incorporated herein by reference, includes a focus adjustment apparatus that tracks the direction of movement of the object image from among a plurality of focus detection areas, and continuously executes focus control regarding the tracked focus areas.
The operation of this type of focus adjustment apparatus is summarized below.
First, the focus adjustment apparatus initializes as the area used for focus control (henceforth called "selected area") the focus detection area positioned in the center of the photographic screen. Next, the focus adjustment apparatus individually measures the defocusing amount, that is, the interval between the image plane of the object image and the imaging surface. Then, the image plane positions are sought for each focus detection area by adding to this defocusing amount the lens positions of the photographic lens.
The focus adjustment apparatus judges the continuity of the previous image plane position and the present image plane position with respect to a series of selected areas. When there is continuity in the image plane positions, the focus adjustment apparatus judges that the object image positioned within the selected areas is in continuous positions within the selected areas. Thus, the focus adjustment apparatus estimates the movement of the image plane position regarding the selected areas, and performs estimated control of the focus.
On the other hand, when there is no continuity in the image plane positions, the focus adjustment apparatus judges that the object image positioned within the selected areas has moved outside the selected areas. Thus, the focus adjustment apparatus reads in the latest image plane position from all the focus detection areas, seeks the difference with the present image plane position in the selected areas, and calculates the image plane velocity.
The calculated image plane velocity is the image plane velocity when it is presumed that the object image previously positioned in the selected areas has moved to other focus detection areas. The focus adjustment apparatus selects from among all of these image plane velocities the one nearest to the previous image plane velocity of the selected areas. Because the area selected in this manner can be judged as the direction of movement of the object image positioned within the selected areas, it is established as the selected area henceforth.
The focus adjustment apparatus continuously performs focus control regarding the selected areas modified in this manner. By repeating such operations, it is possible to continue the focus control while tracking the object image moving on the photographic screen and automatically switching the focus detection areas.
Japanese Laid-Open Patent No. 5-45576, the subject matter of which is incorporated herein by reference, discloses an apparatus that tracks the direction of movement of an object preferentially from the focus detection area, which completes the detection of the defocusing amount first. With such an apparatus, it is possible to begin quickly the tracking of the direction of movement of the object image without relying on the completion of the detection of all the defocusing amounts.
With the focus adjustment apparatus as disclosed in Japanese Laid-Open Patent No. 1-288816, the optimum area for focus control is selected from the defocusing amounts of all the focus detection areas. Therefore, it is not possible to specify the direction of movement of the object image, and the time needed for tracking is long.
With the apparatus as disclosed in Japanese Laid-Open Patent 5-45576, the direction of movement of the object image is tracked in the order of detection of the defocusing amount, without relying on the completion of the detection of all of the defocusing amounts. Nevertheless, the order of detection of the defocusing amount is an order depending on the luminance of the focus detection area. Thus, there is no correlation with the direction of movement of the object image. Therefore, while the tracking would begin quickly, it does not necessarily follow that the direction of movement would be found quickly. Also, because there is no correlation between the order of detection of the defocusing amount and the direction of movement of the object, the possibility of another body accidentally tracked being misidentified as the object is high, resulting in low reliability of the tracking.